Project Summary/Abstract. Because dating violence (DV) and sexual assault (SA) among high school students is a major public health concern, primary prevention is critical. However, there is no comprehensive program to date for high school youth that targets both DV and SA from a bystander intervention framework. Bystander intervention encourages all community members to engage in the prevention of DV and SA rather than focusing on potential victims or perpetrators. Using a cluster randomized control trial, the current study will test the efficacy of an adapted version of Bringing in the Bystander() (BITB) (Eckstein, Moynihan, Banyard, & Plante, 2013), that was originally created for college-age students, with high school youth. The revised program, Bringing in the Bystander-High School Curriculum (BITB-HSC) (Eckstein, Edwards, Leyva, Moynihan, & Banyard, 2014), was developed based on survey and focus group data collected with high school youth and their teachers. In the proposed study, 26 New England high schools will be randomly assigned to the treatment condition group or the control condition group. In the treatment condition schools, students in selected classrooms will receive the seven-session BITB-HSC program, school staff will receive a 60-minute school workshop on DV and SA bystander intervention, and parents will receive an information sheet on DV and SA bystander intervention. Students and school staff in both the treatment condition and control condition schools will include pretest surveys, posttest surveys, five-month follow-up surveys, and one-year follow-up surveys. Multi-level modeling techniques will be used to determine the extent to which student program and school staff workshop participation leads to changes in DV and SA-related attitudes, knowledge, and behaviors among both students and school staff. Multi-level modeling will also be used to examine the mediators (e.g., acquisition of knowledge, change in attitudes) and moderators (e.g., demographics) of program effectiveness. Following the collection of the final, one-year follow-up data, BITB-HSC will be implemented in control condition schools to allow for the assessment of feasibility and fidelity in training teachers and peer educators to deliver the BITB- HSC. The proposed project has the potential to provide practitioners, educators, and researchers with an innovative, evidence-based prevention curriculum that could ultimately reduce the prevalence of DV and SA. Moreover, the proposed project will allow for the identification of mediators and moderators of program effectiveness, something that few studies have examined, but are recognized by experts in the field as a critical aspect of program evaluation. Ultimately, the proposed project has the potential to contribute substantially to the prevention of DV and SA among youth in U.S. society by adding a novel and evidence- based bystander prevention program to the repertoire of existing school-based prevention tools.